Add AI chat with skills management and model provider settings

- Model providers for OpenAI and Anthropic message formats (e.g. via
  Sub2API), with API keys encrypted at rest and never returned
- Skills tree with markdown editing, priorities, drafts, publishing and
  rollback; only published skills enter the chat prompt
- Chat page with tool calls over collected news and Binance USD-M
  klines, a persisted CONTEXT.md, reset, and stop-generation support
- Chat may only create skill drafts and update CONTEXT.md; publishing,
  deleting, moving and priority changes stay with the user

backend/src/alphapulse/core/config.py

@@ -13,6 +13,8 @@ class Settings(BaseSettings):
     admin_username: str
     admin_password_hash: str = ""
     jwt_expire_days: int = 7
+    # Fernet 密钥，加密存库的第三方 API key
+    secret_encryption_key: str
 
 
 @lru_cache